1. Field of the Invention
The present invention relates to an injector.
2. Description of Related Art
With respect to an injector of an internal combustion engine, which includes a needle that is slidable in a valve opening direction and a valve closing direction to open and close an injection hole, industrial research and development has been made to increase a drive force for implementing the valve opening and thereby to improve an injection response. As a result of the research and development, a technique for constructing an actuator using a drive element (e.g., a piezoelectric element or a magnetostrictor), which generates an expansion force, has been proposed to increase the drive force.
An example of a prior art injector 100, which uses such an expansion force, is shown in FIG. 4 (see, for example, WO 2005/075811 corresponding to US2007/0152084A1). The injector 100 includes a needle 102, a piezoelectric actuator 103, a piston 104 and an outer sleeve 106. The needle 102 opens and closes an injection hole 101. The piezoelectric actuator 103 has a piezoelectric element and axially expands and contracts. The piston 104 is axially moved back and forth in response to the contraction and expansion of the piezoelectric actuator 103. The outer sleeve 106 is located radially outward of the piston 104 and slidably supports the piston 104. Furthermore, the outer sleeve 106 defines a fuel pressure chamber 105, a volume of which is increased and decreased in response to the backward movement and forward movement, respectively, of the piston 104.
In this injector 100, the needle 102 is installed in such a manner that the needle 102 receives a fuel pressure of the pressure chamber 105 in the valve opening direction (the upward direction in FIG. 4). That is, the needle 102 is installed such that a distal end surface of a first shaft portion 107 (forming a rear end portion of the needle 102) forms a pressure receiving surface 108, so that the needle 102 receives the fuel pressure toward the rear end side through the pressure receiving surface 108 of the first shaft portion 107. Thereby, the needle 102 defines the pressure chamber 105.
In the injector 100, the high pressure fuel, which is supplied from a fuel supply source (e.g., a common rail), is guided to a nozzle chamber 109. Furthermore, through expansion of the piezoelectric actuator 103, the piston 104 is displaced toward the distal end side to increase the fuel pressure of the pressure chamber 105. In this way, the needle 102 is lifted in the valve opening direction to open the injection hole 101, so that the fuel of the nozzle chamber 109 is injected into a corresponding cylinder from the injection hole 101.
However, in the above injector 100, the first shaft portion 107 is placed radially inward of the piston 104 (i.e., the piston 104 and the first shaft portion 107 are arranged in parallel with each other along the axial direction), so that an outer diameter of the injector 100 is disadvantageously increased. Furthermore, it is difficult to place a stopper, which limits the amount of lift of the needle 102. Furthermore, the displacement direction of the piston 104 and the displacement direction of the needle 102 are opposite to each other. Thus, the relative slide speed of the first shaft portion 107 relative to the piston 104 is relatively large, so that slide wearing, which occurs between the piston 104 and the first shaft portion 107, is prominent.
In order to address the above disadvantage, another injector 100 shown in FIGS. 5A and 5B has been proposed (see, for example, Japanese Unexamined Patent publication No. 2006-152907). In the injector of FIGS. 5A and 5B, the piston 104 and the first shaft portion 107 are arranged in series in the axial direction, so that the outer diameter of the injector 100 can be advantageously reduced. Furthermore, the pressure application surface 111 of the piston 104 and the pressure receiving surface 108 of the firs shaft portion 107 are separated from each other and define different chambers, respectively.
Specifically, in the injector 100 of FIGS. 5A and 5B, the pressure receiving surface 108 defines a control chamber 112, which is separated from the pressure chamber 105, and the pressure receiving surface 108 receives the fuel pressure of the control chamber 112 toward the rear end side of the injector. Furthermore, similar to the injector 100 of FIG. 4, the pressure application surface 111 of the injector 100 of FIGS. 5A and 5B defines the pressure chamber 105 and applies the pressure o the fuel of the pressure chamber 105 toward the distal end side. Also, the pressure chamber 105 and the control chamber 112 are communicated with each other through a communication passage 114, which is provided in a body 113. Furthermore, a fuel chamber 116, which is communicated with a fuel flow passage 115, is formed on a rear end side of the first shaft portion 107.
With the above described structure, in the injector 100, the piston 104 is displaced toward the distal end side by the expansion of the piezoelectric actuator 103 to increase the fuel pressure of the pressure chamber 105, so that the fuel of the increased pressure is supplied to the control chamber 112 to lift the needle 102 in the valve opening direction to open the injection hole 101 and thereby to inject fuel from the injection hole 101 (see FIG. 5B).
With the above described structure of FIGS. 5A and 5B, it is possible to reduce the outer diameter of the injector 100. Also, at the time of lifting of the needle 102, the fuel outflows from the fuel chamber 116, and the portion of the body 113, which is located at the rear end of the fuel chamber 116, functions as a stopper of the needle 102. Furthermore, the needle 102 slidably engages only with the body 113, so that the relative slide speed of the needle 102 is reduced, and thereby the slide wearing can be alleviated.
However, in the injector 100 of FIGS. 5A and 5B, the first shaft portion 107 and the second shaft portion 117 of the needle 102 are both slidably supported in the common body 113. Therefore, in order to lift the needle 102 while maintaining the required fluid tightness of the control chamber 112, a clearance, which is located radially outward of the first and second shaft portions 107, 117, needs to be limited to equal to or smaller than a predetermined value, and concentricity of the first and second shaft portions 107, 117 at the time of displacement of the first and second shaft portions 107, 117 needs to be maintained. Therefore, the needle 102 and the body 113 need to be manufactured with the high accuracy. As a result, in the case of the injector 100 of FIGS. 5A and 5B, the number of manufacturing steps is disadvantageously increased.